Nonwoven fabrics are used to make a variety of products, which desirably have particular levels of softness, strength, uniformity, liquid handling properties such as absorbency, and other physical properties. Such products include towels, industrial wipes, incontinence products, infant care products such as baby diapers, absorbent feminine care products, and garments such as medical apparel. These products are often made with multiple layers of nonwoven fabric to obtain the desired combination of properties. For example, disposable baby diapers made from polymeric nonwoven fabrics may include a liner layer which fits next to the baby's skin and is soft, strong and porous, an impervious outer cover layer which is strong and soft, and one or more interior liquid handling layers which are soft, bulky and absorbent.
Nonwoven fabrics such as the foregoing are commonly made by melt spinning thermoplastic materials. Such fabrics are called spunbond materials and methods for making spunbond polymeric materials are well-known. U.S. Pat. No. 4,692,618 to Dorschner et al. and U.S. Pat. No. 4,340,563 to Appel et al. both disclose methods for making spunbond nonwoven polymeric webs from thermoplastic materials by extruding the thermoplastic material through a spinneret and drawing the extruded material into filaments with a stream of high velocity air to form a random web on a collecting surface. For example, U.S. Pat. No. 3,692,618 to Dorschner et al. discloses a process wherein bundles of polymeric filaments are drawn with a plurality of eductive guns by very high speed air. U.S. Pat. No. 4,340,563 to Appel et al. discloses a process wherein thermoplastic filaments are drawn through a single wide nozzle by a stream of high velocity air. The following patents also disclose typical melt spinning processes: U.S. Pat. No. 3,338,992 to Kinney; U.S. Pat. No. 3,341,394 to Kinney; U.S. Pat. No. 3,502,538 to Levy; U.S. Pat. No. 3,502,763 to Hartmann; U.S. Pat. No. 3,909,009 to Hartmann; U.S. Pat. No 3,542,615 to Dobo et al.; and Canadian Patent Number 803,714 to Harmon.
Spunbond materials with desirable combinations of physical properties, especially combinations of softness, strength and absorbency, have been produced, but limitations have been encountered. For example, for some applications, polymeric materials such as polypropylene may have a desirable level of strength but not a desirable level of softness. On the other hand, materials such as polyethylene may, in some cases, have a desirable level of softness but not a desirable level of strength.
In an effort to produce nonwoven materials having desirable combinations of physical properties, multicomponent or bicomponent nonwoven polymeric fabrics have been developed. Methods for making bicomponent nonwoven materials are wellknown and are disclosed in patents such as U.S. Pat. No. Re. 30,955 of U.S. Pat. No. 4,068,036 to Stanistreet, U.S. Pat. No. 3,423,266 to Davies et al., and U.S. Pat. No. 3,595,731 to Davies et al. A bicomponent nonwoven polymeric fabric is made from polymeric fibers or filaments including first and second polymeric components which remain distinct. As used herein, filaments mean continuous strands of material and fibers mean cut or discontinuous strands having a definite length. The first and subsequent components of multicomponent filaments are arranged in substantially distinct zones across the cross-section of the filaments and extend continuously along the length of the filaments. Typically, one component exhibits different properties than the other so that the filaments exhibit properties of the two components. For example, one component may be polypropylene which is relatively strong and the other component may be polyethylene which is relatively soft. The end result is a strong yet soft nonwoven fabric.
U.S. Pat. No. 3,423,266 to Davies et al. and U.S. Pat. No. 3,595,731 to Davies et al. disclose methods for melt spinning bicomponent filaments to form nonwoven polymeric fabrics. The nonwoven webs may be formed by cutting the meltspun filaments into staple fibers and then forming a bonded carded web or by laying the continuous bicomponent filaments onto a forming surface and thereafter bonding the web.
To increase the bulk or fullness of the bicomponent nonwoven webs for improved fluid management performance or for enhanced "cloth-like" feel of the webs, the bicomponent filaments or fibers are often crimped. As disclosed in U.S. Pat. Nos. 3,595,731 and 3,423,266 to Davies et al., bicomponent filaments may be mechanically crimped and the resultant fibers formed into a nonwoven web or, if the appropriate polymers are used, a latent helical crimp produced in bicomponent fibers or filaments may be activated by heat treatment of the formed web. This heat treatment is used to activate the helical crimp in the fibers or filaments after the fibers or filaments have been formed into a nonwoven web.
One problem with fabrics made from helically crimped bicomponent filaments or fibers is that the web, when heat treated to activate the latent helical crimp, shrinks irregularly and becomes non-uniform. This problem is addressed in published European Patent Application Number 0,391,260 to Taiju et al. This reference discloses a method for melt spinning continuous bicomponent filaments to form a nonwoven web wherein an air stream is blown against the formed web from below the moving forming surface to float the web above the forming surface and disentangle the web from the forming surface before the web is heat treated to develop crimps and thermally bond the web. Although this process claims to produce a substantially uniform and highly crimped nonwoven fabric, it suffers from serious drawbacks in that it requires an additional process step, namely, floating the web above the forming surface, and is slow due to the long heating and bonding step which takes more than one minute. Such drawbacks add cost to the process making it impracticable for commercial use.
Therefore, there is a need for nonwoven materials having desirable levels of physical properties such as softness, strength, uniformity and absorbency, and efficient and economical methods for making the same.